This invention relates to optical transmission systems in which optical wave guides interconnect optical terminals for conducting optical signals therebetween, at least one element of the optical transmission system exhibiting birefringence and being susceptible to degradation of the optical signal by polarization mode dispersion.
The presence of polarization mode dispersion can be a limiting factor in the design of optical transmission systems, particularly those providing long haul transmission of signal data streams of 10 Gb/sec or more over single mode fibers of the order of 100 kilometers in length. Although such fibers are nominally "single mode", propagation is generally characterized by two orthogonally polarized HE.sub.11 modes for which slightly different group velocities exist in the presence of birefringence. Consequently, signal pulses launched into one end of the fiber become degraded by the effect of pulse energy being periodically coupled between the fast and slow propagation modes, the resulting dispersion in the received signal typically being characterized by a polarization mode dispersion parameter M which for long fibers is proportional to .sqroot.L where L is the fiber length.
The amount of polarization mode dispersion varies from fiber to fiber, being dependent upon the amount of intrinsic birefringence associated with core asymmetry or frozen-in stress and extrinsic birefringence associated for example with cable induced stress, fiber bends or twists.
Optical transmission systems which currently use 10 Gb/s signal data are able to tolerate polarization mode dispersion of the order of 0.2 pico seconds per .sqroot.(kilometer). The next generation of optical transmission systems expected to utilize 40 Gb/s data transmission will however be more likely to be limited by the effects of polarization mode dispersion, particularly in systems which incorporate cross connected networks of fibers so that the route taken by an optical signal can be any one of a number of possible routes utilizing different fibers within the same or different cable, each with individual properties.
Existing methods of measuring polarization mode dispersion typically require a series of measurements based on the input of test signals at discreet wavelengths as described by B. L. Heffner, IEEE Photonics Technology Letters, September 1992 pp 1066-1069. Such measurement techniques require complex test equipment and cannot be implemented at the operating frequency while the optical transmission system is in normal use.
It is also known from S. C. Rashleigh and R. Ulrich, Optics Letters; Vol 3, No 2; August 1978, to measure the amount of polarization mode dispersion in short fibers from the depolarization of a test signal constituted by broadbandwidth light.
A fully automated interferrometric PMD (polarization mode dispersion) measurement is also disclosed by Y. Namihira et al, OFMC 93, Torino, 1993, the technique being applied to fiber amplifiers, optical fibers and other fiber optic devices.
It is also known from U.S. Pat. No. 5,473,457 to compensate for PMD in an optical fibre by passing the received optical signal through a polarization maintaining fibre arranged to apply an equal and opposite dispersion, any misalignment between the respective principal axes of the fibres being compensated by means of a polarization controller. The fibre is however assumed to have a predetermined PMD which remains constant.